Preparation of low–power loss MgCuZn ferrites using the microwave sintering method

A reduction in size and weight of power supplies can be achieved by using switched mode or resonant concepts. For the voltage conversion several circuit designs are in use with ferrites as transformer core materials. Transformer ferrites must show low energy losses at high induction levels at higher and higher frequencies [1–3]. This requires development of new ferrite materials with constantly improving loss characteristics. To reduce the power loss, both hysteresis loss and eddy current loss should be suppressed by the available preparation technique. In this connection, it was found that the microwave sintering technique markedly enhanced the densification rate at lower sintering temperature of the material. In this method of preparation, the material can absorb the microwaves and self-generate heat [4–6]. Recently, we have developed low temperature sintered MgCuZn ferrites used for multilayer chip inductors [7]. These ferrites were conventionally sintered at 910 ◦C/12 hr. In continuation of this work, microwave sintered MgCuZn ferrites were selected for this investigation. The total power loss was measured. Pure (99.99%) MgO, ZnO, CuO, and Fe2O3 powders with different particle size were used for the preparation. The α-Fe2O3 coarse particles (average diameter 0.13 μm) were synthesized by the oxidation of Fe3O4 particles at 800 ◦C. Industrial grade particles of magnesium oxide, zinc oxide, and copper oxide were used as the coarse particles with the average diameters of 0.8, 0.15, and 0.75 μm, respectively. Fine particles of magnesium oxide, zinc oxide, and copper oxide were synthesized by the thermal decomposition of the oxalates of MgO, ZnO, and CuO at 400 ◦C in air. These powders were mixed together with wet attrition milling. Their molar ratio was adjusted to obtain the composition Mg0.58Cu0.12Zn0.30Fe2O4, and then the powders were made into two batches. One batch (Batch-I) of the powders was calcined at 800 ◦C/4 hr using the conventional ceramic method. The second batch (Batch-II) of the powders was calcined at 800 ◦C/30 min using a microwave furnace. Then the two batches were reground by wet ball milling for 10 hr to form slurry. To these powders, 2 wt% poly-vinyl-alcohol was added as a binder. The granules were compacted at a pressure of 190 MPa for 10 min, into pelletes (12-mm diameter, 3-mm thickness) and rods (60-mm length, 12-mm diameter) and toroids (12-mm outer diameter, 6-mm inner diameter, 4-mm thickness). The specimens were heated up to 400 ◦C to remove the binder and the lubricant. The first batch of the samples was conventionally sintered (CS) at 870 ◦C/6 hr, 890 ◦C/6 hr, 910 ◦C/6 hr, and 930 ◦C/6 hr in air at atmospheric pressure. The second batch (Batch-II) of the samples was sintered using the microwave sintering (MS) method at 870 ◦C/20 min, 890 ◦C/20 min, 910 ◦C/20 min, and 930 ◦C/20 min in air at atmospheric pressure. The microwave sintering process was carried out using a specially designed applicator which consists of a domestic microwave oven having an output power level tunable up to a maximum of 800 W and an operating frequency of 2.45 GHz. The structure and microstructure of the sintered materials were examined using X-ray diffractometry and scanning electron microscopy (SEM), respectively. The bulk density was measured by the Archimedes method. The room temperature magnetic properties were obtained by recording hysteresis loops with the help of a vibrating sample magnetometer (VSM). The initial permeability (μi) of the toroidal sample was measured using the HP 4191A impedance analyzer in the frequency range of 1–100 MHz. The power loss, Pt, was measured at 100 kHz–1 MHz with the flux density of 50 mT with the B-H/Z analyzer (HPE5060 A). Fig. 1a and b, c and d shows the X-ray diffraction patterns for the few typical samples prepared by conventional and microwave sintering methods, respectively. It can be seen from the figure that the samples contain only single phase, irrespective of whether the materials were densified by the conventional or microwave sintering techniques. The average size of the ferrite grain, geometrically estimated from SEM photographs, and the results are presented in Table I. The grain size for the present samples varies from 3 to 16 μm irrespective of sintering methods. The grain size of MS samples is smaller than that of CS ferrites. It can be seen from the table that the densification rate has been significantly increased in the microwave sintering process. In both preparation techniques, the spinel ferrite formation is completed at a low temperature of 870 ◦C. Conventional sintering process requires at least 16 hr to reach the sintering temperature of 870 ◦C and soaking time of 6 hr to obtain a sample with 91% of theoretical density (TD). Similarly, the density of the samples increased to 94% of TD with an increase of the sintering temperature from 870 to 930 ◦C, and time taken was 24 hr. In contrast, the microwave sintering process needs only 30 min to reach 870 ◦C, and a soaking period of 20 min to obtain a sample with the density as high as 94% of TD. The